1. Field of the Invention
The present invention relates to a DC-DC converter of a step-up type for boosting a DC voltage.
2. Description of the Prior Art
To generate a voltage greater than the voltage supplied from a power supply, such as a battery, or the like, and supply a voltage for driving a light emitting device or liquid crystal display device, a step-up converter having high power conversion efficiency has been used. The step-up converter mainly formed by an inductor, a switch, a rectifier and a smoother. In such a step-up converter, when the switch is ON, an input voltage is applied to the inductor so that the inductor is excited. When the switch is OFF, a current flows from the inductor to the smoother through the rectifier. Then, an increased DC voltage is supplied from the smoother to a load.
FIG. 6 is a circuit diagram showing a conventional step-up converter. The step-up converter shown in FIG. 6 is described in Japanese Laid-Open Patent Publication No. 10-225104, and the structure and operation of this step-up converter are described below.
The conventional step-up converter shown in FIG. 6 includes an inductor 121, an N-channel FET 122, a diode 123, a P-channel FET 125, an output capacitor 124, and a control circuit 126. The inductor 121 receives input voltage Vi at one end. The N-channel FET 122 has a source connected to the ground and a drain connected to the other end of the inductor 121 and serves as a switch. The diode 123 rectifies a current output from the other end of the inductor 121. The P-channel FET 125 has a drain connected to the other end of the inductor 121 and an anode of the diode 123 and a source connected to a cathode of the diode 123. One end of the output capacitor 124 is connected to the cathode of the diode 123 and the source of the P-channel FET 125, and the other end of the output capacitor 124 is connected to the ground. The output capacitor 124 serves as a smoother. The control circuit 126 controls the operation of the gate electrode of the N-channel FET 122 and the operation of the gate electrode of the P-channel FET 125. The control circuit 126 supplies pulse PA to the gate electrode of the N-channel FET 122 and pulse PB to the gate electrode of the P-channel FET 125. The output capacitor 124 outputs output voltage Vo. Output voltage Vo is input to the control circuit 126.
The control circuit 126 controls pulses PA and PB such that output voltage Vo equals a desired value. Specifically, a period where pulse PA is at H level, i.e., a period where the N-channel FET 122 is ON, is necessarily within a period where pulse PB is at H level, i.e., a period where the P-channel FET 125 is OFF. Therefore, the N-channel FET 122 and the P-channel FET 125 are never ON at the same time. A circuit operation of such a conventional step-up converter is described below.
When pulse PA is at H level and the N-channel FET 122 is ON, input voltage Vi is applied to the inductor 121. This applied voltage Vi excites the inductor 121.
Then, when pulse PA transitions to L level so that the N-channel FET 122 is turned OFF, energy accumulated in the inductor 121 is discharged through the diode 123 in the form of a current for charging the output capacitor 124. After a very short delay period, pulse PB transitions from H level to L level so that the P-channel FET 125 is turned ON. Accordingly, a current from the inductor 121 flows through the P-channel FET 125.
Then, when pulse PB transitions to H level, the P-channel FET 125 is turned OFF so that energy accumulated in the inductor 121 is again discharged through the diode 123 to the output side. After a very short delay period, pulse PA transitions to H level so that the N-channel FET 122 is turned ON. Accordingly, voltage Vi is applied to the inductor 121 so that voltage Vi excites the inductor 121. By repeating the operation described hereinabove, input voltage Vi is converted to output voltage Vo, whereby power is transferred to the output side.
The control circuit 126 controls the ON/OFF period ratio of the N-channel FET 122 by controlling the pulse widths of pulses PA and PB. With this, the amount of energy accumulated in the inductor 121 and the amount of energy discharged from the inductor 121 are adjusted such that output voltage Vo is controlled to equal a desired value. Herein, the ON resistance of the P-channel FET 125 is sufficiently small, and a voltage drop caused by a current which flows when the P-channel FET 125 is ON is smaller than the forward voltage of the diode 123. Therefore, the power loss caused by rectification is reduced, and the power conversion efficiency of the step-up converter is improved. Although the P-channel FET 125 and the diode 123 are in parallel to each other in FIG. 6, the body diode of the P-channel FET 125 can substitute for the diode 123.
Japanese Laid-Open Patent Publication No. 10-225104 discloses a technique wherein a voltage detection circuit is provided for detecting whether or not output voltage Vo has sufficiently increased and, when output voltage Vo has not sufficiently increased, the operation of the P-channel FET 125 is stopped.
As described above, when a switching element like the P-channel FET 125 is used as a rectifier, the power loss caused by rectification is reduced, and the power conversion efficiency of the step-up converter is improved. However, when the switching element is ON, a current can flow in both directions. Therefore, in order to drive the switching element to perform a rectification function, in general, the switching element is turned OFF when a reverse current flows from the output side to the input side. Hereinafter, a switching element which is used as a rectifier, e.g., the P-channel FET 125, is referred to as a rectification switch.
A driving circuit for driving a rectifier switch which prevents a reverse current is disclosed in Japanese Laid-Open PCT National Phase Publication No. 60-502135. FIG. 7 shows a simplified version of the driving circuit shown in FIG. 2 of Japanese Laid-Open PCT National Phase Publication No. 60-502135.
In the driving circuit shown in FIG. 7, the voltage difference at both ends of a rectification switch 130 formed by a P-channel FET is sensed by a comparator 131, and the output of the comparator 131 is supplied to the gate electrode of the rectification switch 130.
When a current flows in the forward direction from the input side to the output side as indicated by an arrow in FIG. 7, a voltage drop occurs in the rectification switch 130 so that the output of the comparator 131 transitions to L level, whereby the rectification switch 130 is turned ON. A voltage drop due to the ON resistance of the rectification switch 130 maintains the output of the comparator 131 at L level. Therefore, the rectification switch 130 is kept ON so long as the current flows in the forward direction. However, when the forward current of the rectification switch 130 decreases below zero, the voltage drop due to the ON resistance of the rectification switch 130 becomes negative so that the output of the comparator 131 is inverted to H level. As a result, the rectification switch 130 is turned OFF. Thus, the driving circuit turns the rectification switch 130 ON only when the current flows in the forward direction, thereby preventing a reverse current from flowing through the rectification switch 130.